$ perl -le '$a="foo"; my @n = (3,2,1); print for sort {$a<=>$b} @n; print $a'
1
2
3
foo

##</code><code>##

$ perl -le 'my $a; my @n = (3,2,1); print for sort {$a<=>$b} @n'
Can't use "my $a" in sort comparison at -e line 1. 

##</code><code>##

$ perl -le 'my $a; my @n = (3,2,1); print for sort {$::a<=>$::b} @n;'
1
2
3